The innate immune system forms the first line of specific host defense against invading microbial pathogens. Integral to the innate immune response are Pattern Recognition Receptors (PRRs) that recognize conserved determinants expressed by a broad range of pathogenic microorganisms. Among these are the Toll-like receptors, which promote inflammatory signaling and phagocyte recruitment in response to pathogen recognition, and several families of phagocytic PRRs, including scavenger receptors and C-type lectin receptors that mediate phagocytic clearance of microbes from infected tissues. Each receptor family contains multiple members that recognize a unique spectrum of microbial components, and collectively they provide protection against a wide range of infectious organisms. We have identified a novel pattern recognition receptor, BAI1, that is expressed on macrophages and recognizes the surface lipopolysaccharide (LPS) of Gram-negative bacteria using an array of thrombospondin repeats (TSRs) in its extracellular domain. TSRs in several secreted proteins have been shown to bind microbial products, but this is the first evidence of a transmembrane receptor that uses TSRs to mediate the phagocytic clearance of specific pathogens. Binding of bacteria to BAI1 triggers phagocytosis through a mechanism involving activation of the small GTPase Rac by the Elmo/Dock180 GEF complex, which interacts directly with BAI1. An emerging theme is that pattern recognition receptors often cooperate with each other to enhance downstream signaling responses. Our preliminary data suggest that BAI1 acts synergistically with the Toll-like receptor TLR4 to promote the synthesis of inflammatory cytokines/chemokines. BAI1 is an orphan GPCR-like receptor and apart from its ability to activate Rac, almost nothing is known about how it transmits signals to the cell interior or which signaling pathways are activated in response to BAI1 ligation. The overall goal of the proposed research is to define the role of BAI1 in host defense against bacterial infection. In Aim 1, we will define the mode of interaction of LPS with the BAI1 TSRs, using a combination of biochemical, biophysical and structural approaches. In Aim 2, we will identify components of the signaling machinery that interact with BAI1, define the signaling pathways activated in response to LPS or bacterial activation of the receptor, and elucidate the mechanisms of crosstalk between BAI1 and TLR4. In Aim 3, we will characterize the role of BAI1 in host defense in vivo, using a mouse model of Salmonella infection. Together these studies will provide unique structural, biochemical and physiological insights into a new class of pattern recognition receptor that may have important functions in the innate immune response to bacterial pathogens.